Helical scanning computed tomography with tracking x-ray source

ABSTRACT

A CT apparatus reduces errors in projection data acquired in helical scanning. The imaged object moves concurrently along a translation axis and the x-ray beam is periodically translated with the imaged object so as to subtend a single predetermined volume element during the acquisition of one projection set of data for a first slice. The x-ray beam then returns to its starting position and tracks a second predetermined volume element within a next slice. The x-ray beam may be translated by moving the focal point or a collimator or a combination of both.

BACKGROUND OF THE INVENTION

This invention relates to computed tomography (CT) systems andspecifically to a helical scanning CT system in which the imaged objectis concurrently translated during the acquisition of tomographicprojections.

In a computed tomography system, an x-ray source is collimated to form afan beam with a defined fan beam angle. The fan beam is typicallyoriented to lie within the x-y plane of a Cartesian coordinate system,termed the "gantry plane", and is transmitted through an imaged objectto an x-ray detector array oriented within the gantry plane. Thedetector array is comprised of an array of detector elements each ofwhich measures the intensity of transmitted radiation along a rayprojected from the x-ray source to the particular detector element. Theintensity of the transmitted radiation is dependent on the attenuationof the x-ray beam along that ray by the imaged object.

The center of the fan beam and its direction of the fan beam isidentified by a fan beam axis.

The x-ray source and detector array may be rotated on a gantry withinthe gantry plane and around a center of rotation within the imagedobject so that the angle at which the fan beam axis intersects theimaged object may be changed. At each gantry angle, a projection isacquired comprised of the intensity signals from each detector element.The gantry is then rotated to a new angle and the process is repeated tocollect a number of projections along a number of gantry angles to forma tomographic projection set. The acquired tomographic projection setsare typically stored in numerical form for later computer processing to"reconstruct" a slice image according to reconstruction algorithms knownin the art. A projection set of fan beam projections may bereconstructed directly into an image by means of fan beam reconstructiontechniques, or the intensity data of the projections may be sorted intoparallel beams and reconstructed according to parallel beamreconstruction techniques. The reconstructed tomographic images may bedisplayed on a conventional CRT tube or may be converted to a filmrecord by means of a computer controlled camera.

A typical computed tomographic study involves the acquisition of aseries of "slices" of an imaged object, each slice parallel to thegantry plane and having a slice thickness dictated by the width of thedetector array, the size of the focal spot, the collimation and thegeometry of the system. Each successive slice is displaced incrementallyalong a z-axis, perpendicular to the x and y axes, so as to provide athird spatial dimension of information. A radiologist may visualize thisthird dimension by viewing the slice images in order of position alongthe z-axis, or the numerical data comprising the set of reconstructedslices may be compiled by computer programs to produce shaded,perspective representations of the imaged object in three dimensions.

As the resolving power of computed tomography methods increases,additional slices are required in the z-dimension. The time and expenseof a tomographic study increases with the number of slices required.Also, the longer scan times necessary to acquire more slices increasesthe discomfort to the patient who must remain nearly motionless topreserve the fidelity of the tomographic reconstructions. Accordingly,there is considerable interest in reducing the time required to obtain aslice series.

The time required to collect the data for a series of slices depends inpart on four components: a) the time required to accelerate the gantryto scanning speed, b) the time required to obtain a complete tomographicprojection set, c) the time required to decelerate the gantry, and d)the time required to reposition the patient in the z-axis for the nextslice. Reducing the time required to obtain a full slice series may beaccomplished by reducing the time required to complete any of these foursteps.

The time required for acceleration and deceleration of the gantry (a andc) may be avoided in tomographic systems that use slip rings rather thancables to communicate with the gantry. The slip rings permit continuousrotation of the gantry and avoid the need for acceleration anddeceleration. Hereafter, it will be assumed that the CT systemsdiscussed are equipped with slip rings or the equivalent to permitcontinuous rotation.

The time required to acquire the tomographic data set (b) is moredifficult to reduce. Present CT scanners require on the order of one totwo seconds to acquire the projection set for one slice. This scan timemay be reduced by rotating the gantry at a faster speed. However, ahigher gantry speed, in general, will decrease the signal-to-noise ratioof the acquired data by the square root of the factor of rotational rateincrease. This may be overcome to some extent by increasing theradiation output of the x-ray tube, but is subject to the power limitsof such devices.

Finally, a reduction in patient repositioning time (d) may beaccomplished by translating the patient in the z-axis concurrently withthe rotation of the gantry. The combination of continuous patienttranslation along the z-axis during the rotation of the gantry andacquisition of projection data has been termed "helical scanning" andrefers to the apparent path of a point on the gantry with respect to areference point on the imaged body. As used herein, "helical scanning"shall refer generally to the use of continuous translation of thepatient or imaged object during the acquisition of tomographic imagingdata, and "constant z-axis scanning" shall refer to the acquisition ofthe tomographic data set without translation of the patient or imagedobject during the acquisition period.

Continuous translation of the imaged object during scanning shortens thetotal scanning time required for the acquisition of a given number ofslices by eliminating the length of time normally required forrepositioning the patient between scans. However, helical scanningintroduces certain errors in the acquired tomographic projection sets.The mathematics of tomographic reconstruction assumes that thetomographic projection set is acquired along a constant z-axis sliceplane. The helical scan path clearly deviates from this condition andthis deviation results in image artifacts in the reconstructed sliceimage if there is any significant change in the object in the z-axis.The severity of the image artifacts depends generally on the "helixoffset" in the projection data, measured as the z-axis differencebetween the scanned volume elements of the imaged object and the z axisvalue of the desired slice plane. Errors resulting from helical scanningwill be referred to collectively as "skew" errors.

Several methods have been used to reduce skew errors in helicalscanning. A first approach disclosed in U.S. Pat. No. 4,630,202 issuedDec. 16, 1986, reduces the pitch of the helical scan and then averagesthe projection data of consecutive 360° tomographic projection sets. Theeffect is equivalent to using a detector array with a larger width alongthe z axis, which also moves less in the z direction during a rotationof the gantry, i.e. with a lesser scanning pitch. Skew errors arereduced using this method, but at the expense of additional scanningtime necessitated by the lower scanning pitch. Thus, this methodreduces, to some extent, the advantages to be gained by helicalscanning.

Skew errors at the ends of the tomographic projection set may be reducedin conjunction with this approach by changing the weighting of the lastand first projections of the consecutive 360° tomographic projectionsets in the "averaging" process to give greater weight to the projectionclosest to the slice plane.

A second approach disclosed in U.S. Pat. No. 4,789,929 issued Dec. 6,1988, also applies weighting to the projections of combined, consecutive360° tomographic projection sets, but the weighting is a function of thehelix offset of each projection at the given gantry angle. This approachof interpolating over 720° generally increases partial volume artifacts.Partial volume artifacts are image artifacts arising when certain volumeelements of the imaged object contribute to only some of the projectionsof the projection set.

A third approach, described in co-pending U.S. Patent application Ser.No. 07/435,980, entitled: "Extrapolative Reconstruction Method forHelical Scanning" and assigned to the same assignee as the presentinvention, uses a half-scanning technique to reduce the table motionduring the acquisition of each slice. Projection data is acquired over360° of gantry rotation and interpolated to a slice plane. The reducedgantry motion corresponds to reduced table motion and hence certainhelical scanning artifacts are reduced.

SUMMARY OF THE INVENTION

The present invention reduces skew error by translating the x-ray beamwith translation of the imaged object. Specifically, the imaged objectis translated concurrently along a translation axis while an opposedx-ray generator and x-ray detector in opposition around the imaged bodyare rotated around the imaged object in a gantry plane substantiallyperpendicular to the translation axis. The x-ray generator projects abeam of x-rays through the imaged object and during a first periodsweeps the beam along the translation axis following the translation ofthe imaged object. During a second period, the x-ray generator returnsto its initial orientation, moving in a second direction along thetranslation axis counter to the translation of the imaged object. Thesweeping of the x-ray beam may be such as to center the x-ray beamaround a predetermined volume element in the imaged object and to trackthat volume element during the first period. The movement of the x-raybeam may be by moving a collimator or the x-ray source or both.

It is one object of the invention to reduce the helix offset of theprojection data acquired in helical scanning without interrupting thecontinuous motion of the imaged object and gantry. The sweeping of thex-ray beam serves to counteract the effective motion of the imagedobject during the acquisition of each projection set.

In one embodiment, the x-ray generator includes a stationary x-raysource and a movable collimator that may be rapidly repositioned inresponse to movement of the imaged object.

It is thus another object of the invention to provide a simple means ofsweeping the x-ray beam with motion of the imaged object.

The foregoing and other objects and advantages of the invention willappear from the following description. In the description, reference ismade to the accompanying drawings which form a part hereof and in whichthere is shown by way of illustration, a preferred embodiment of theinvention. Such embodiment does not necessarily represent the full scopeof the invention, however, and reference must be made therefore to theclaims herein for interpreting the scope of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation CT system gantry including an x-raysource and x-ray detector as may be used with the present invention;

FIG. 2 is a schematic illustration of the imaged object of FIG. 1showing the relative orientation of the gantry and gantry axis withrespect to the imaged object for helical scanning. The pitch of thehelical scanning is exaggerated for clarity;

FIG. 3 is a perspective view of the collimator assembly of the presentinvention;

FIG. 4 (a) and (b) are cross-sectional views of the mandrel of thecollimator of FIG. 3 showing orientation of the mandrel for thick andthin fan beams respectively;

FIG. 5 is a block diagram showing the control system for the collimatorand x-ray focal point of FIG. 3 according to the present invention;

FIG. 6 is a simplified cross-sectional view of the path of the x-ray fanbeam, taken along line 5--5 in FIG. 1, with the x-ray tube anode, thecollimator and the detector array exaggerated for clarity and showing afirst method of reducing helix offset requiring only movement of thecollimator;

FIG. 7 is a cross-sectional view, similar to that of FIG. 6, of a secondmethod of reducing helix offset requiring movement of the collimator andthe x-ray focal point but reducing movement of the illuminated area ofthe detector;

FIG. 8 is a cross-sectional view, similar to that of FIG. 6, a thirdmethod of reducing helix offset requiring movement of the collimator andthe x-ray focal point but further reducing skew error;

FIG. 9 is a cross sectional view, similar to that of FIG. 6, of a fourthmethod of reducing helix offset requiring only movement of the x-ray;

FIG. 10 is graph of the z-axis displacement with time for thecollimator, the illumination area of the detector and the imaged elementvolume for the method of FIG. 6;

FIG. 11 is graph of the z-axis displacement with time for thecollimator, the illumination area of the detector and the imaged elementvolume for the method of FIG. 7;

FIG. 12 is graph of the z-axis displacement with time for thecollimator, the illumination area of the detector and the imaged elementvolume for the method of FIG. 8;

FIG. 13 is graph of the z-axis displacement with time for thecollimator, the illumination area of the detector and the imaged elementvolume for the method of FIG. 9; and

FIG. 14 is an exaggerated cross-sectional view of the imaged objecttaken along 5--5 in FIG. 1, showing a single slice thickness, theeffective thickness for helical scanning and for helical scanning withreduced helix offset per the methods of FIGS. 6-8.

FIG. 15 is a schematic representation similar to FIG. 1 showing thedetermination of l₃ ^(') and 1'₂ for offset translation axes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, a gantry 20, such as may be used in a "thirdgeneration" computed tomography (CT) scanner, includes an x-ray source10 collimated by collimator 38 to project a fan beam of x-rays 22through imaged object 12 to detector array 14. The x-ray source 10 anddetector array 14 rotate on the gantry 20 about center of rotation 13.The rotation of the gantry 20, as indicated by arrow 28 is within agantry plane 60, aligned with the x-y plane of a Cartesian coordinatesystem.

The imaged object 12 rests on table 17 which is radio-translucent so asnot to interfere with the imaging process. Table 17 may be controlled sothat its upper surface translates along the z axis perpendicular to thex-y imaging plane, moving the imaged object 12 across the gantry plane60.

The detector array 14 is comprised of a number of detector elements 16,organized within the gantry plane 60, which together detect theprojected image produced by the attenuated transmission of x-raysthrough the imaged object 12.

The fan beam 22 emanates from a focal point 26 in the x-ray source 10and is directed along a fan beam axis 23 centered within the fan beam22. The fan beam angle, measured along the broad face of the fan beam,is larger than the angle subtended by the imaged object 12 so that twoperipheral beams 24 of the fan beam 22 are transmitted past the bodywithout substantial attenuation. These peripheral beams 24 are receivedby peripheral detector elements 18 within the detector array 14.

Referring to FIG. 6, the x-ray source 10 includes an anode 29 positionwithin an evacuated glass envelope and rotated about anode shaft 25 forheat dispersion. A stream of electrons from a cathode (not shown) isaccelerated against the face of the anode 29 to produce the x-ray beam19. The face of the anode 29 is beveled with respect to the fan beamaxis 23 so that radial displacement of the electron beam by focussingplates, (not shown) as is known in the art, will produce a z-axisdisplacement of the focal point 26. The amount of this displacement maybe controlled by x-ray controller 62.

Referring to FIG. 2, the angular position θ of the gantry 20 along thez-axis with respect to the imaged object 12 is shown by arrows 11. Thez-axis position of the imaged object 12 with respect to the imagingplane 60 changes constantly during the acquisition of each tomographicprojection set. Accordingly, arrows 11 are shifted along a helix withinthe imaged object 12 along the z-axis. The pitch of the helix will bereferred to as the scanning pitch. The z-axis distance from the center 9of the slice being acquired to the volume elements 7 intercepting thefan beam 22 is termed the "helix offset" of that volume element. In thepresent invention the fan beam axis 23 may be shifted along the z-axisduring the helical scan to reduce the helix offset as will be described.

Referring to FIG. 3, uncollimated x-rays 19 radiating from the focalpoint 26 in the x-ray source 10 (not shown in FIG. 3) are formed into acoarse fan beam 21 by primary aperture 40. As is understood in the art,the uncollimated x-rays 19 are produced by a high voltage x-ray tubetypically including a rotating anode (not shown) receiving a high energybeam of electrons and re-emitting x-ray radiation. The coarse fan beam21 is collimated into fan beam 22 by means of collimator 38.

Referring generally to FIGS. 3, 4(a) and 4(b), collimator 38 iscomprised of a cylindrical x-ray absorbing molybdenum mandrel 39 heldwithin the coarse fan beam 21 on bearings 42 allowing the mandrel 39 torotate along its axis. A plurality of tapered slots 41 are cut throughthe mandrel's diameter and extend along the length of the mandrel 39.The slots 41 are cut at varying angles about the mandrel's axis topermit rotation of the mandrel 39 to bring one such slot 41 intoalignment with the coarse fan beam 21 so as to permit the passage ofsome rays of the coarse fan beam 21 through the slot 41 to form fan beam22.

Referring to FIG. 4(a) and 4(b), the tapered slots 41 are of varyingwidth and hence the rotation of the mandrel 39 allows the width of thefan beam 22 to be varied between narrow (1 mm) as shown in FIG. 4(b) andwide (10 mm) as shown in FIG. 4(b). The slots 41 ensure dimensionalaccuracy and repeatability of the fan beam 22.

The slots 41 are tapered so that the entrance aperture 43 of each slot41, when orientated with respect to the coarse fan beam 21, is widerthan the exit aperture 45. The exit aperture 45 defines the width of thefan beam 22 and the extra width of the entrance aperture 43 preventseither edge of the entrance aperture 43 from blocking the coarse fanbeam 21 during small angular rotation of the mandrel 39. Such smallrotations of the mandrel 39 are used to provide adjustment of the z-axisposition of the fan beam 22 as will be discused in detail below.

Referring again to FIG. 3, a positioning motor 48 is connected to oneend of the mandrel 39 by flexible coupling 50. The other end of themandrel 39 is attached to a position encoder 46 which allows accuratepositioning of the mandrel by motor 48. Fan beam angle shutters 44 ateither ends of the mandrel 39 control the fan beam angle.

Referring now to FIG. 5, the control system of a CT scanner, suitablefor use with the present invention, has gantry associated controlmodules 60 which include: x-ray controller 62 which provides power andtiming signals to the x-ray source 10, and which in certain embodimentsof the invention, controls the position of the focal point 26;collimator controller 64 which controls the rotation of the collimator38; gantry motor controller 66 which controls the rotational speed andposition of the gantry 20; and the data acquisition system 68 whichreceives projection data from the detector array 14 and converts thedata to digital words for later computer processing.

The gantry associated control modules 60 communicate with the x-ray tube10, collimator 38 and detector 14 via slip rings 61. It will berecognized that direct cabling using a take up reel may be substitutedfor the slip rings 61 for a limited gantry rotation system.

The x-ray controller 62, the collimator controller 64 and the gantrymotor 66 controller are connected to a computer 70. The computer 70 is ageneral purpose minicomputer such as the Data General Eclipse MV/7800Cand may be programmed to synchronize the rotation of the gantry 20 withthe position of the fan beam 22 per the present invention as will bedescribed in detail below.

The data acquisition system 68 is connected to image reconstructor 72which receives sampled and digitized signals from the detector array 14via the data acquisition system 68 to perform high speed imagereconstruction according to methods known in the art. The imagereconstructor 72 may be an array processor such as is manufactured byStar Technologies of Virginia.

The speed and position of table 17 along the z-axis is communicated toand controlled by computer 70 through of table motor controller 74. Thecomputer 70 receives commands and scanning parameters via operatorconsole 76 which is generally a CRT display and keyboard which allows anoperator to enter parameters for the scan and to display thereconstructed image and other information from the computer 70. A massstorage device 78 provides a means for storing operating programs forthe CT imaging system, as well as image data for future reference by theoperator.

Referring now to FIG. 6, the z-axis position of the exit aperture 45 ofthe collimator 38 may be adjusted so that the fan beam 22, as indicatedby fan beam axis 23, diverges from the gantry plane 60 in the z-axisdimension during the acquisition of the first projection of a projectionset. The amount of divergence of the fan beam axis 23 from the gantryplane 60 is such that a volume element 7 at position 80 within a sliceand moving toward the gantry axis 60 with motion of table 17, isintersected by the fan beam axis 23.

The position of the table 17 during the acquisition of the projectionset is determined by the table motor controller 74. The collimator 38 ascontrolled by the collimator controller 64 is coordinated by computer 70with the position of table 17 so that during the movement of the table17 and imaged object 12, the fan beam axis 23 is swept as to constantlyintercept volume element 7.

As the projections of each projection set are acquired, during a periodT₁, the imaged object 12 is translated along the z-axis with respect tothe imaging plane 60 so that volume element ultimately moves to position82 at the last projection of the projection set. Typically, the amountof translation will be equal to the slice thickness w.

At the completion of the acquisition of the projection set, the exitaperture 45 of the collimator 38 is returned to the position it had atthe start of the projection set, moving in the opposite direction,during a period T₂, so that the fan beam axis 23 intercepts a new volumeelement in a new slice. The new volume element has the same relativeposition 80 with respect to the gantry plane 60 as did volume element 7at the start of the acquisition of the previous projection set.Preferably, positions 80 and 82 are located symmetrically about thegantry plane 60 so as to reduce the maximum deviation of the fan beamaxis 23 from the gantry plane 60 during any acquisition.

At the halfway point in the acquisition of the projection set, the focalpoint 26, the center line of the exit aperture 45 of the collimator 38,fan beam axis 23 and the center of illumination of the detector array 14will be perfectly aligned with the gantry plane 60. At all other times,these various points may deviate from the gantry plane 60. The measuresof the deviation of the center line of the exit aperture 45 of thecollimator 38, the volume element intersected by fan beam axis 23, andthe center of illumination of the detector array 14 from the gantryplane will be termed C_(z), V_(z), and D_(z) respectively.

For the first described embodiment shown in FIG. 6, F_(z), the positionof the focal point 26 with respect to the gantry plane 60 is constantand zero.

Referring to FIG. 10, during the first period T₁ of the acquisition of aprojection set, the displacement of the collimator C_(z) will increaseso that the fan beam axis 23 tracks the movement of the volume element7. For large values of 1₂ and 1₃ and small values of slice thickness w,the relationship between the collimator displacement C_(z) and thedisplacement V_(z) of the fan beam axis 23 with axis of translation 84of the volume element 7 is: ##EQU1## where l₁ is the distance betweenthe focal point 26 and the exit aperture 45 of the collimator 38 and l₂is the distance between the exit aperture 45 and the translation axis 84of the volume element 7. Accordingly, during the first period T₁, theposition of table 17 as determined via the table motor controller 74,determines the position of the exit aperture 45 after suitable scalingby computer 70 as given in equation (1) above.

During a second time period T₂, immediately after the first time periodT₁, the exit aperture 45 is returned to the position it had at the startof that acquisition of projections to prepare for acquisition of asecond projection set. Preferably this period T₂ is made a short aspossible by moving the collimator 38 at its maximum speed. During thisreturn period T_(z), no projection data is taken and the x-ray fan beam22 may be decreased in intensity according to any of several methodsknown in the art such as decreasing current flow to the x-ray tube orshuttering the x-ray beam 19.

It will be noted that the displacement D_(z) of the fan beam axis 23with respect to the surface of the detector array 14 will be larger thanthe displacement V_(z) according to the following ratio: ##EQU2## wherel₃ is the distance between the axis of translation 84 of the volumeelement 7 and the exposed surface of detector array 14. Generally, thedetector elements 16 of detector array 14 exhibit a change ofsensitivity as a function of the z-axis position of their illumination.Hence a variation in D_(z) will introduce some variation into theprojections measurements. This variation may be corrected by using theperipheral beams 24 and peripheral detector elements 18 to provide areference for correcting sensitivity variation according to compensationmethods understood in the art. One such method is given in U.S. Pat. No.4,559,639 hereby incorporated by reference.

In a second embodiment shown in FIG. 7 and 11, both the x-ray focalpoint 26 and the exit aperture 45 of the collimator 38 are moved.Movement of the x-ray focal point 26 is accomplished by refocussing theelectron beam on the anode 29 as has been previously described or byphysical translation of the x-ray source 10 under the control of servomotors. The measure of the deviation of the focal point 26 from thegantry plane 60 will be termed: F_(z). Referring to

FIG. 11, in this second embodiment, the intersection D_(z) of the fanbeam axis 23 on the detector array 14 is maintained constant (at zerodisplacement) by controlling the displacement F_(z) of the focal pointand the displacement C_(z) of the exit aperture 45 with respect to thedisplacement V_(z) of the volume element as follows: ##EQU3##

Referring to FIG. 14, the acquisition volume 86 within the imaged object12 over which projection data is acquired in a non-helical scan will beapproximately one half of acquisition volume of a helical scan: assumingthat the scanning pitch times the rotation for one projection set isapproximately equal to the slice thickness w. The present invention, asdescribed in the above two embodiments, enlarges the acquisition volumeover the non-helical acquisition volume 86 by flanking volumes 88 whichare outwardly conically concave. This increase in acquisition volumerepresented by volumes 88 increases the helix offset of the projectiondata slightly but much less than that produced by helical scanning whichadds areas 90 to effectively double the acquisition volume. In general,the greater the distance l₁ +l₂ in comparison to the radius of the imageobject 12 about the translation axis 84, the less the flanking volume 88and thus the less the helix offset of the data.

Referring to FIGS. 14 and 8, a third embodiment of the inventioneliminates the flanking volumes 88 and produces an acquisition volume 86identical to that of non-helical scanning. Referring to FIG. 12, thedisplacement D_(z) of the collimator 38 and F_(z) of the focal point 26are set equal to the displacement V_(z) of the volume element 7. The fanbeam axis 23 is thus maintained parallel to the gantry plane 60 at alltimes.

In a fourth embodiment, shown in FIGS. 9 and 13, the displacement C_(z)of the exit aperture 45 of the collimator 38 is fixed (and equal tozero) and the displacement F_(z) of the focal point 26 is adjustedaccording to the following relationship: ##EQU4##

The acquisition volume (not shown) for this method and the amount ofdisplacement D_(z) of the fan beam axis 23 on the detector array 14 willbe greater than the comparable quantities for the previously describedmethod, for CT systems of similar dimensions as a result of the greaterangular divergence of the fan beam axis 23 from the gantry plane 60necessary to track a given volume element 7 without movement of thecollimator 38.

For each of the above embodiments, the projection data for volumeelements near volume element 7 on the translation axis 84, there will belittle helix offset. To the contrary, the volume elements removed fromvolume element 7 and the translation axis 84 will have increasingamounts of helix offset for greater values of x and y as dictated by theangle of the fan beam axis 23 with respect to the gantry plane 60.

For this reason, it may be desirable to position the volume element 7and the translation axis 84 near internal structures of interest withinimaged object 12.

The translation axis 84 will normally intersect the center of rotation13 of the gantry 20. The center of rotation 13 and the translation axis84 may both be moved within the imaged object simply by adjusting theheight of table 17. Alternatively, the translation axis 84 may be movedindependently from the center of rotation 13 by adjusting the fan beamangle 23 as a function of gantry rotation 28. This is most easilyaccomplished by modifying the apparent value of l₂ and l₃ used bycomputer 70 in the above embodiments as a function of gantry angle θ asfollows:

    l.sub.2 '=l.sub.2 -cos(θ+α) (Δ)          (6)

    l.sub.3 '=l.sub.3 +cos(θ+α) (Δ)          (7)

where α is the angle with respect to the center of rotation 13 betweenthe volume of interest and gantry angle θ=O, Δ is the distance betweenthe volume of interest and the center of gantry rotation 13, and l₂ 'and l₃ ' are substituted into the above equations in place of l₂ and l₃respectively.

For the embodiments shown in FIGS. 6, 7, and 9, it will be understoodthat the amount of helix offset, reduced as it is, also varies as afunction of the order of the projection within the projection set. Forexample, when the starting and ending positions 80 and 82 of the volumeof interest 7 are symmetrically displaced about the gantry plane 60, thecentermost projections will have no helix offset and the starting andending projections will have the most helix offset. For this reason, itis desirable to weight the projections so as to deemphasize the startingand ending projections and to emphasize the centermost projections ofthe projection set. Such weighting systems are disclosed in co-pendingapplication 07/440,531 entitled:"Method for Reducing Patient TranslationArtifacts in Tomographic Imaging" filed Nov. 22, 1989. Finally, for thefirst, second, and forth embodiments, where the center of illuminationof the detector 14 changes during the acquisition of projections, it isimportant that the detector 14 be sufficiently wide so as to alwaysreceive the entire fan beam 22.

Many modifications and variations of the preferred embodiment which willstill be within the spirit and scope of the invention will be apparentto those with ordinary skill in the art. For example, the collimator maybe of a conventional bladed design. Further it will be apparent thatthis method is applicable to so called "forth generation" CT machineswhere the detector array 14 is stationary and may surround the imagedobject 12. Clearly the the x-ray tube and collimator may be alsomechanically translated and tipped as a single unit. Finally, the tablemotion need not be constant during the acquisition of successiveprojection sets but may be slowed, for example, during the period T₂when the fan beam 22 repositions itself at a starting position. In orderto apprise the public of the various embodiments that may fall withinthe scope of the invention, the following claims are made:

We claim:
 1. A method of acquiring tomographic projection data of animaged object comprising:supporting and translating the imaged objectconcurrently along a translation axis; projecting a beam of x-rays froman x-ray generator through the imaged object and alternately sweepingthe beam along the translation axis in a first direction withtranslation of the imaged object during a first period, and in a seconddirection along the translation axis but counter to the translation ofthe imaged object during a second period; receiving the beam from thex-ray generator with an x-ray detector array after it passes through theimaged object; and holding the x-ray generator and x-ray detector inopposition around the imaged body and concurrently rotating the samearound a center of rotation and the imaged object, in an gantry planesubstantially perpendicular to the translation axis.
 2. The method asrecited in claim 1 wherein the x-ray exposure of the imaged object bythe x-ray beam during the first period is greater than the x-rayexposure of the imaged object by the x-ray beam during the secondperiod.
 3. The method as recited in claim 2 wherein the intensity of thex-ray beam is reduced during the second period.
 4. The method as recitedin claim 1 wherein the sweeping of the x-ray beam during the firstperiod is such as to maintain the beam centered on a predeterminedvolume element on the translation axis.
 5. The method recited in claim 4wherein the x-ray generator comprises:a fixed x-ray source; and acollimator having an aperture movable along the translation axis.
 6. Themethod recited in claim 5 wherein the aperture of the collimator iscontrolled according to the following equation: ##EQU5## where V_(z) isthe distance along the translation axis between the predetermined volumeelement and the gantry plane;C_(z) is the distance along the translationaxis between the center of the collimator and the gantry plane; l₁ isthe distance between the x-ray source and the collimator; and l₂ is thedistance between the collimator and the translation axis.
 7. The methodrecited in claim 5 wherein the aperture of the collimator is controlledaccording to the following equation gantry angle θ as follows: ##EQU6##where

    l.sub.2 '=l.sub. -cos(θ+α) (Δ)

and where θ is the gantry angle; V_(z) is the distance along thetranslation axis between the predetermined volume element and the gantryplane; C_(z) is the distance along the translation axis between thecenter of the collimator and the gantry plane; α is the angle, withrespect to the center of rotation, between the volume of interest andgantry angle θ=O; Δ is the distance between the volume of interest andthe center of gantry rotation; l₁ is the distance between the x-raysource and the collimator; and l₂ is the distance between the collimatorand the translation axis.
 8. The method recited in claim 4 wherein thex-ray generator comprises an x-ray source having a focal point movablealong the translation axis and collimator having a aperture movablealong the translation axis.
 9. The method recited in claim 8 wherein thefocal point and aperture of the collimator are controlled according tothe following equations: ##EQU7## where V_(z) is the distance along thetranslation axis between the predetermined volume element and the gantryplane;F_(z) is the distance along the translation axis between x-raysource and the gantry plane; C_(z) is the distance along the translationaxis between the center of the collimator and the gantry plane; l₁ isthe distance between the x-ray source and the collimator; l₂ is thedistance between the collimator and the translation axis; and l₃ is thedistance between the translation axis and the x-ray detector.
 10. Themethod recited in claim 8 wherein the focal point and the aperture ofthe collimator are controlled according to the following equations:##EQU8## where

    l.sub.2 '=l.sub.2 -cos(74 +α) (Δ)

    l.sub.3 '=l.sub.3 +cos(θ+α) (Δ)

and where θ is the gantry angle; α is the angle, with respect to thecenter of rotation, between the volume of interest and gantry angle θ=O;Δ is the distance between the volume of interest and the center ofgantry rotation; V_(z) is the distance along the translation axisbetween the predetermined volume element and the gantry plane; F_(z) isthe distance along the translation axis between x-ray source and thegantry plane; C_(z) is the distance along the translation axis betweenthe center of the collimator and the gantry plane; l₁ is the distancebetween the x-ray source and the collimator; l₂ is the distance betweenthe collimator and the translation axis; and l₃ is the distance betweenthe translation axis and the x-ray detector.
 11. The method recited inclaim 8 wherein the focal point and the aperture of the collimator arecontrolled according to the following equation:

    F.sub.z =C.sub.z =V.sub.z

where V_(z) is the distance along the translation axis between thepredetermined volume element and the gantry plane; F_(z) is the distancealong the translation axis between x-ray source and the gantry plane;and C_(z) is the distance along the translation axis between the centerof the collimator and the gantry plane.
 12. The method recited in claim4 wherein the x-ray generator comprises an x-ray source having a focalpoint movable along translation axis and a fixed collimator aperture.13. The method recited in claim 12 wherein the focal point is controlledaccording to the following equation: ##EQU9## where F_(z) is thedistance along the translation axis between x-ray source and the gantryplane;V_(z) is the distance along the translation axis between thepredetermined volume element and the gantry plane; l₁ is the distancebetween the x-ray source and the collimator; and l₂ is the distancebetween the collimator and the translation axis.
 14. The method recitedin claim 10 wherein the focal point is controlled according to thefollowing equation: ##EQU10## where

    l.sub.2 '=l.sub.2 -cos(θ+α) (Δ)

and where θ is the gantry angle; α is the angle, with respect to thecenter of rotation, between the volume of interest and gantry angle θ=O;Δ is the distance between the volume of interest and the center ofgantry rotation; F_(z) is the distance along the translation axisbetween x-ray source and the gantry plane; V_(z) is the distance alongthe translation axis between the predetermined volume element and thegantry plane; l₁ is the distance between the x-ray source and thecollimator; and l₂ is the distance between the collimator and thetranslation axis.